The present invention is related to a servo control apparatus which is constituted in a multiple shaft manner by a converter unit and a plurality of inverter units.
As conventional techniques for detecting ground faults, for example, a patent publication 1 is opened. The conventional techniques will now be explained with reference to drawings. FIG. 2 is a block diagram for showing an arrangement of one conventional technique. In FIG. 2, reference numeral 101 indicates an AC power supply which is grounded, and reference numeral 102 shows an AC/DC converting circuit (rectifying circuit and converter) which is arranged by a diode bridge and a smoothing capacitor CB. Reference numeral 103 indicates a DC/AC converting circuit (inverter) in which a bridge circuit is arranged by employing respective arms (namely, in U phase, V phase, W phase, X phase, Y phase, and Z phase) of transistors and diodes. The DC/AC converting circuit 103 outputs 3-phase AC power having an output frequency and an output voltage in correspondence with a frequency command. The output of the DC/AC converting circuit 103 is connected to an induction motor 104, so that the induction motor 104 is driven in a variable speed. A load current of the Z phase of this induction motor 104 is detected by a current detector 105, and then, an output signal of the current detector 105 constitutes a signal “i” which is directly in proportional to the load current. This current “i” is continuously compared with each other by an overcurrent detecting circuit 106 so as to perform an overcurrent protection. That is, in the overcurrent protection, when the overcurrent is detected by the overcurrent detecting circuit 106, the operation of the DC/AC converting circuit 103 is stopped. Further, the current “i” is inputted to a comparator 111 of a ground fault detecting circuit 109. In the comparator 111, the signal “i” is compared with a reference value vs2, and when the signal “i” exceeds the reference value vs2, the comparator 111 outputs such a signal of an output “1.” A drive control circuit 108 outputs a signal “b” to a base drive circuit 107 during a normal operation. This signal “b” turns ON/OFF the transistors of the respective arms as to the U, V, W, X, Y, and Z phases. When the above-explained signal “b” is not outputted, the drive control circuit 108 further outputs aground fault detecting instruction signal “a” in response to a necessity of investigating a ground fault condition. When the signal “a” is inputted to a ground fault detecting-purpose base driving circuit 110, this ground fault detecting-purpose base driving circuit 110 outputs a drive signal with respect to the transistor of the Z phase for a predetermined time period. At this time, when a ground fault happens to occur in the output, as indicated in FIG. 2, a ground fault current “IG” flows through the transistor of the Z phase. At this time, the current detector 105 apparently detects a current “iG” of the Z phase to produce a signal “i.” The comparator 111 compares this signal “i” with the reference value vs2, and outputs a signal “c” of an output “1” when this signal “i” exceeds the reference value vs2. The signal “a” is entered to one input terminal of an AND circuit 113, and the above-explained signal “c” is entered from the comparator 111 to the other input terminal of the AND circuit 13. When both the signal “a” and the signal “c” are “1”, the AND circuit 113 outputs a signal “d” of an output “1.” The output signal “d” of the AND circuit 113 is inputted to a latch circuit 114 so as to be latched by the latch circuit 114. An output signal “g” of this latch circuit 114 prohibits the operation of the base drive circuit 107, and furthermore, is outputted to a display device 112, so that the display device 112 displays an occurrence of the ground fault and produces an alarm. Also, in addition, this output signal “g” of the latch circuit 114 is outputted as a ground fault detection signal to an external unit.
Also, another conventional technique is shown in FIG. 3. FIG. 3 is such an example that a servo control apparatus of multiple shafts such as a robot is constructed. In an application example to a robot, motors are built in the robot, and the servo control apparatus is connected to a robot control apparatus. Wiring lines wired to the motors are penetrated through the robot, and are moved in response to operations of the robot. originally, cables having anti-bending characteristics are used as these wiring lines. However, in connection with such a fact that action ranges of the robot are expanded, the bending ratio of the cables is increased. As a result, there is such an actual fact that occurring ratios of mechanical wearing of the cables and of disconnections of the cables become very high, as compared with those of other systems. In robot systems, ground fault detecting functions are necessary required. In FIG. 3, reference numeral 1 shows a power supply, reference numeral 2 indicates a breaker, reference numeral 3 represents a converter unit, reference numeral 4 shows a servo control circuit, and reference numeral 5 shows an inverter unit. Also, reference numeral 6 denotes a ground fault detecting circuit, reference numeral 7 shows a current detector, and reference numeral 8 denotes a motor. Since the current detector 7 is inserted in the output of the inverter unit 5, a zero-phase-sequence current is detected. As a method of detecting a zero-phase-sequence current, a sum of currents which flow through wired lines is detected. If no ground fault occurs, then the sum of currents becomes zero. When the ground fault is detected by the ground fault detecting circuit 6, the ground fault detecting circuit 6 outputs a ground fault signal to the servo-control circuit 4, and thus, the servo control circuit 4 is designed to turn OFF a power element of the inverter unit 5 and the breaker 2.
[Patent Document 1]
JP-A-5-328739 (FIG. 1)
However, in the conventional technique, since the zero-phase-sequence current detectors are used in the outputs of the inverter unit, plural sets of these zero-phase-sequence detectors and plural sets of ground fault current detecting circuits are required, the total quantities of which are equal to a total number of the shafts. Also, in order to turn OFF the breaker, the ground fault signals derived from the respective shafts must be received. As a result, plural sets of the wiring lines are required for the total quantity of these shafts. Among current servo control systems for robots, in particular, a strong demand as to compact sizes is made, and volumes occupied by servo control apparatus may become a problem and higher cost is required.